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Current transformer related problems and Solutions

Current transformer related problems and Solutions

2021-12-09

If the secondary load impedance of current transformer exceeds its allowable secondary load impedance Why does accuracy decline?


The secondary load impedance of current transformer has a great influence on the accuracy of current transformer. This is because if the secondary load impedance of the current transformer increases a lot and exceeds the allowable secondary load impedance, the value of the excitation current will greatly increase and the iron core will enter the saturation state. In this case, a large part of the primary current will be used to provide the excitation current, thus greatly increasing the error of the transformer, Its accuracy decreases.

current transformer

The current transformer for differential protection requires a good iron core, and the iron core section should be increased. Why?


When the system operates normally or there is a short circuit outside the differential protection range, the current value and phase of the current transformers at both ends of the differential protection are the same, and there should be no current flowing into the differential relay. However, in fact, the characteristics of the two sets of current transformers cannot be exactly the same, the excitation current is different, the secondary current will not be equal, and the unbalanced current will flow through the relay. In order to reduce the unbalanced current, the structure of current transformer must be improved to make it unsaturated, or special silicon steel sheet with small loss must be selected to make the iron core, and the cross section of the iron core must be increased.


What are the ways of wiring the secondary winding of current transformer?

According to different requirements of relay protection and automatic device, the secondary winding of current transformer usually has the following wiring modes:

(1) complete (three-phase) star connection;

(2) incomplete (two-phase) star connection;

(3) triangular connection;

(4) three phase parallel connection to obtain zero sequence current wiring;

(5) two phase difference wiring;

(6) one phase is connected in series with two current transformers;

(7) one phase is connected in parallel with two current transformers.


What is the wiring coefficient of current transformer? What is the effect of wiring factor?

The ratio of the current passing through the relay to the secondary current of the current transformer is called the wiring coefficient of the current transformer, that is, KC = IK / I2

IK -- current flowing into relay;

12 -- secondary current flowing into current transformer;

The connection coefficient is an important parameter in the setting calculation of relay protection. The connection coefficient should be considered in the calculation of the action value of various current protection measuring elements.


What is reverse charging of voltage transformer? What is the impact on the protection device?

Charging the uncharged bus through the secondary side of the voltage transformer is called reverse charging. For example, if the transformation ratio of 220kV voltage transformer is 2200, even if the primary bus without power failure is not grounded, its impedance (including bus capacitance and insulation resistance) is large, which is assumed to be 1m Ω, but the impedance seen from the secondary measurement of voltage transformer is only 1000000 / (2200) 2 = 0.2 Ω, which is almost short-circuit, so the reverse charging current is large (the reverse charging current mainly depends on the cable resistance and the leakage reactance of two voltage transformers), It will cause the small switch at the secondary side of the voltage transformer to trip or the fuse to fuse during operation, resulting in the loss of voltage of the protection device during operation, which may cause misoperation or refusal of operation of the protection device.


Installation of zero sequence current transformer

(1) The cable head and the support of zero sequence current transformer shall be reliably isolated with insulation.

(2) When single-phase grounding occurs, the grounding current may flow not only in the ground, but also along the cable sheath. In order to prevent misoperation in case of single-phase grounding fault outside the area, the grounding wire of cable head shall pass through the zero sequence current transformer and then be grounded.


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